Alkanes — preparation (Wurtz, Kolbe electrolysis, hydrogenation), properties, free-radical halogenation (Cl₂ - Br₂): Foundations

Before you touch Wurtz, Kolbe, or halogenation, you must be able to read every squiggle the parent note throws at you. Below, each symbol is built from nothing: plain words first, then the picture, then why the topic can't live without it.

1. The atoms and the plain bond

The dash in $C - C$ or $C - H$ is a single bond: two electrons shared between two atoms, one from each. Picture two balls holding one shared pair of hands.

Figure — Alkanes — preparation (Wurtz, Kolbe electrolysis, hydrogenation), properties, free-radical halogenation (Cl₂ - Br₂)

2. Reading a molecular formula: $C_{n} H_{2 n + 2}$

WHY that exact count $2 n + 2$ , and why do we need a formula at all? Because it is the fingerprint of "fully saturated." Line up $n$ carbons in a chain: the two end carbons each have 3 spare hands, and each middle carbon has 2 spare hands. Count them and you always land on $2 n + 2$ hydrogens. If a molecule has fewer H than $2 n + 2$ , some carbons must be sharing a double bond — it is not an alkane. So the formula is your test for "is this thing saturated?"

3. $R$ — the "rest of the molecule" shorthand

Picture $C H_{3} - C H_{2} - C l$ . The interesting part is the $- C l$ ; everything to its left ( $C H_{3} C H_{2}$ ) we lazily call $R$ . So the whole thing is $R - C l$ , or in symbol soup $R - X$ .

4. The dot: $s p^{3}$ geometry and the shape of a chain

WHY the topic mentions $s p^{3}$ at all: it explains two later facts. (1) The chain is a zig-zag, not a straight line, which is why a branched alkane can curl into a compact near-sphere (matters for boiling points in §Properties of the parent). (2) There are no flat $π$ -electron clouds sticking above the atoms for an attacker to grab — reinforcing why alkanes are inert. You don't need to derive $s p^{3}$ here; just picture the pyramid. Deeper electron-donation effects live in Hyperconjugation.

5. The single most important new symbol: the radical dot $R^{∙}$

Compare the three ways a bond can break, side by side:

Homolysis (even split): $A : B \to A^{∙} + B^{∙}$ . Each keeps one electron. This makes radicals.
Heterolysis (uneven split): $A : B \to A^{+} + : B^{-}$ . One side keeps both electrons. This makes ions — a positive $A^{+}$ and a negative $B^{-}$ (with a lone pair, drawn $:$ ).

6. Charge signs and the arrows

WHY these live above the arrow: the same reactants can do nothing or explode depending on conditions. $C H_{4} + C l_{2}$ sit quietly in the dark; add $h ν$ and they react. The condition is half the reaction.

7. The anode, oxidation, and $C O_{2}$

WHY the topic needs "anode = oxidation = lose electron": Kolbe only works because the carboxylate is dragged to the anode and stripped of an electron, becoming a radical $R C O O^{∙}$ . Without the electrode vocabulary, step 2 of the mechanism is unreadable.

C_{n} H_{2 n + 2}

and find the H count for any

n

Yes —

H = 2 n + 2

; e.g.

n = 5

gives

C_{5} H_{12}

I know what the dash in

C - C

physically means

A single bond = one shared pair of electrons.

I can say what

R

and

X

stand for

R

= any alkyl group (rest of the chain);

X

= any halogen

F, C l, B r, I

I can explain the bullet

^{∙}

A free radical — one unpaired electron from an even (homolytic) bond split.

I can tell homolysis from heterolysis

Homolysis splits evenly → two radicals; heterolysis splits unevenly → two ions.

I know why alkanes are unreactive

Saturated, non-polar, no

π

-electrons or lone pairs — only radicals can attack.

I can read

h ν

Δ

e^{-}

Light (photon), heat, and a single electron respectively.

I know what happens at the anode

Oxidation — particles lose electrons (needed for Kolbe).

I know why

C O_{2}

leaving drives Kolbe

C O_{2}

is very stable; forming it is energetically downhill.

I can define symmetrical vs mixed products

Symmetrical = same group both sides (clean); mixed = cross-coupling gives a messy mixture.

Foundations — Alkanes — preparation (Wurtz, Kolbe electrolysis, hydrogenation), properties, free-radical halogenation (Cl₂ - Br₂)

1. The atoms and the plain bond

2. Reading a molecular formula: $C_{n} H_{2 n + 2}$

3. $R$ — the "rest of the molecule" shorthand

4. The dot: $s p^{3}$ geometry and the shape of a chain

5. The single most important new symbol: the radical dot $R^{∙}$

6. Charge signs and the arrows

7. The anode, oxidation, and $C O_{2}$

8. The two "why it feels smart but fails" ideas you must pre-load

Prerequisite map

Equipment checklist